Various schemes have been explored to improve wireless transmission, in particular mobile telephony. In current practice, workers in the field employ systems that transmit data simultaneously along a number of channels, issuing from two or more transmit antennas and being received in one or more receive antennas.
Modulated wireless signals from a transmitter reach a receiver by a number of propagation paths. The characteristics of each propagation path vary over time and between one another subject to factors such as fading, multipath, and different signal to interference and noise ratio (SINR). Multiple transmit and/or receive antennas may be used to provide diversity that insures against adverse effects on any of these paths, because the likelihood of correctly receiving a transmission increases with the number of transmit antennas so long as one propagation path is not merely a (linear) combination of the others. This diversity-oriented approach accommodates both space-time coding and space-frequency coding, as well as a mix thereof (sometimes called space-frequency-time coding), and may include knowledge of channel state at the transmitter. This approach is fully realizable with only one receiver antenna, and additional receiver antennas may be simply used to add receiver diversity gain, or to facilitate capacity improvements.
While multiple receive and/or multiple transmit antennas—giving rise to multiple input multiple output (MIMO) channels—have been successfully employed to enhance diversity, they also allow a substantial increase in communication capacity as compared to non-MIMO systems. Under certain conditions, that increase is linearly related to the number of transmit or receive antennas. The resulting MIMO channel may be considered as a number of independent channels, the number being at most the number of transmit antennas. Each of the independent channels is also referred to as a spatial subchannel of the overall MIMO channel, and corresponds to one spatial dimension.
A bit sequence is sent by modulating a signal, according to constellation points, onto either a single carrier wave to assume discrete values of a signal parameter, or a set of subcarriers, in the case of orthogonal frequency division multiplexing (OFDM). While increasing the number of available constellation points allows increased data rates over a given bandwidth, the increase necessarily increases error frequency at the decoder because adjacent constellation points are closer in proximity to one another as compared to a constellation with fewer points. Trellis coded modulation (TCM) is one coding technique wherein modulation and coding are combined in a manner that reduces error rate by restricting transitions between adjacent constellation points, and thereby avoiding bandwidth expansion. Other coding techniques employ block coding, and include low-density parity check (LDPC) codes.
In an uncoded system, the relevant minimum distance between adjacent constellation points is merely the Euclidean distance. A fundamental concept of TCM systems is that transitions between adjacent constellation points are not allowed during the process of adding redundancy for the purpose of forward error correction. TCM systems allow transitions only between non-adjacent points, so that the minimum Euclidean distance between points in an allowed transition, is greater than the Euclidean distance between two nearest adjacent points. TCM systems can thus increase coding gain without increasing bandwidth.
Regarding the use of coded modulation in fading channels, conventional use of TCM (alone, or via an outer TCM and a concatenated inner code) have proven unable to achieve a diversity order of more than about three in fast fading environments, and more than about five in space-time bit interleaved coded modulation schemes. What is needed in the art is a method and apparatus to increase or maximize the diversity order in a fast fading environment, especially using multiple transmit antennas, across which coding may be performed simultaneously, or jointly. Prior art solutions using bit interleaved coded modulation schemes dispose an interleaver between an encoder and modulator, which separates coding from modulation, and undermines certain capacity advantages attainable by using multiple transmit antennas.
These and other features, aspects, and advantages of embodiments of the present invention will become apparent with reference to the following description, and in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention.